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Bike Gear Ratio Calculator

Calculate gear ratios, gear inches, and development (meters of rollout) for road, mountain, and gravel bikes based on chainring, cassette, and tire size.

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About

Cyclists use gear ratios to optimize performance for specific terrains. A climber needs low gear inches to maintain cadence on steep gradients, while a track sprinter requires high development for maximum speed. This tool calculates three critical metrics: Gear Ratio, Gear Inches, and Development (rollout).

Accuracy depends on the precise wheel diameter. ISO/ETRTO standards provide a baseline, but tire width affects the final circumference. Comparing these values helps mechanics and riders select the correct cassette or chainring size before purchasing components. Mechanical advantage determines the force required at the pedal to rotate the wheel.

cycling gear inches cadence fixie mtb

Formulas

The core calculation for development relies on the tire circumference:

Development = TfrontTrear × Ctire

Where T represents the number of teeth and C is the circumference. Gear inches, a traditional imperial measure, uses the wheel diameter:

GI = TfrontTrear × Dwheel

Reference Data

MetricFormulaUnitTypical Range (Road)
Gear RatioChainringCogRatio1.0 to 4.5
Gear InchesRatio × Wheel DiameterInches30 to 120
DevelopmentRatio × CircumferenceMeters2.5 to 10.0
Gain RatioRatio × (RadiusCrank)Ratio2.0 to 9.0

Frequently Asked Questions

Gear Inches is an abstract number representing the diameter of the wheel if the crank were directly attached to it (like a penny-farthing). Development is a physical measurement representing the distance the bike travels for one full rotation of the pedals. Development is generally preferred in metric-using countries and scientific analysis.
The rim diameter (e.g., 622mm for 700c) is fixed, but the tire adds height. A 700x23c tire has a smaller circumference (~2096mm) than a 700x45c tire (~2240mm). Ignoring this difference introduces an error of approximately 5-7% in rollout data.
Gravel riding involves steep, loose surfaces where traction is limited. A ratio below 1:1 (e.g., 40t front, 50t rear) allows the rider to spin at a high cadence without applying excessive torque that would cause the rear wheel to slip.
Crank length does not affect Gear Inches or Development. However, it does affect "Gain Ratio", which accounts for the leverage arm length. A longer crank provides more leverage but requires a larger circle of foot movement.